GolfPro2301
With regards to your top vents. I would have a typical cap on both of them to protect then from rain getting in. I would also have a butterfly style damper in each one so that you can adjust them. In addition I don't see if there is an intake vent at the bottom. It should also be adjustible if possible. You want your smoke to flow by and gently kiss the meat. Too much air flow and you will have trouble maintaining heat and too little air flow will make things taste like an ashtray. (not that I've ever tasted an ash tray
)
I've built a few watt burners. They have all been PID controlled. My cabinets are slightly larger but my "go to" element is a 220v 3000 watt oven element. They are readily available and economically priced. Here's a picture of one of them along with a link to a post with he build. There is a wiring diagram there as well if you are interested.
http://forum.bradleysmoker.com/index.php?topic=16075.msg192278#msg192278
With regards to the type or model of PID, you want one with an SSR output. Auber does have some that have a built in SSR but they are built for a 10 amp max load. This narrows it down to 2 different models. One is a 30 step programable unit, in other words you can preprogram temps to ramp up or down over time. The other one is a manual type, in other words you set the temp to what you want. Both these controllers have 2 temperature based alarms built into them. You could use one of these to turn your fan on at a preset temperature. As far as wiring the display lights in, you certainly can do that and it's really not that dificult, however, both these controllers also have indicator lights on the display for both the alarms as well as when the heating element is engaged. Although I will say additional lights on the control panel do add the cool factor.
As someone else mentioned, you may want to run a Heat Sink on your SSR. If you follow the link to my build you will see I used a small computor style fan to force air over the heat sink in my setup.
I'm not sure about the purpose of the cooling fan you mentioned is. I have used high temp fan/blower for a circulation fan inside the smoker to try to balance out the temps inside the cabinet. The built in alarm relays are rated for 3 amps so you will need to keep that in mind.
Here is a link to the 2 controllers...
Non-programable SSR output -
http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=3
Programable SSR output -
http://www.auberins.com/index.php?main_page=product_info&cPath=1&products_id=4
Here is the Thermocouple that I use in mine. I don't use a plug, so I wire them directly mainly because my controllers are dedicated to the smoker they are connected to.
http://www.auberins.com/index.php?main_page=product_info&cPath=20_3&products_id=101
The 25 amp SSR that you linked to would be fine for your setup. The nice thing about the SSRs is it doesn't matter if you want to switch 110v or 220v, the same SSR will work. If you are ever going to add more heat you could consider getting the 40 amp SSR because it's only a couple dollars more.
http://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=9
There are 2 different types of heat sinks. One that mounts outside the controller cabinet and one that would be typically mounted inside the controller cabinet. If you opt for the one that mounts inside the controller cabinet you will have to run a cooling fan for it. The various heat sinks are towards the bottom of the page in this link.
http://www.auberins.com/index.php?main_page=index&cPath=2_30
Hope that helps.
Mike